Anodes for Electrowinning of Metals
Electrowinning involves the recovery of a metal, usually from its ore, by dissolving a metallic compound in a suitable electrolyte and reducing it electrochemically through passage of a direct electric current. Increasingly more metals are being produced by electrowinning because of the stringent air pollution restraints on the more conventional concentrating, smelting and electrorefining processes.
Anode material suitable for electrowinning of metals has been a source of difficulty. The requirements are insolubility in the sulfate electrolytes, resistance to the mechanical and chemical effects of oxygen liberated on the anode surface, low oxygen overvoltage and resistance to breakage in handling.
At present, lead alloy anodes have been used in most plants for the electrowinning of metals such as copper, nickel, zinc, etc. A particular problem with lead anodes is prevention of lead transfer from the anode to the electrowon metal deposited on the cathode.
Copper electrowinning consists of using an aqueous solution of copper sulfate containing free sulfuric acid, electrolyzing it with an insoluble anode, and depositing its copper content as pure copper on the cathode. Oxygen is released at the anode. Copper electrowinning cells are open concrete tanks lined with plastic or rubber, approximately 1 meter (m) across, 1 m deep and 5-15 m long. Electrodes measuring about 1 m.times.1 m hang vertically at intervals of about 50 millimeters (mm). The electrodes are arranged so that they are alternately anodic and cathodic, and all anodes and cathodes in a single tank are usually connected in parallel. Copper is deposited on cathode starting sheets of pure copper, stainless steel, or titanium. The electrolyte contains from 25 to 40 grams per liter (g/L) of copper and from 100 to 160 g/L of sulfuric acid. Electrowinning of copper is carried out at current densities from 160 to 270 amperes per square meter (A/m.sup.2) and electrolyte temperature 30-50 centigrade (.degree. C.) (see Encyclopedia of materials science and engineering/edited by M. B. Bever.--Oxford: Cambridge, Mass.: Pergamon; MIT Press, 1986, pp. 1444-1445).
Conventional lead anodes for copper electrowinning are stabilized with antimony, calcium and/or tin, and by adding cobalt to the electrolyte, both of which inhibit electrocorrosion of lead. However, copper obtained by electrowinning using lead alloy anodes is not pure enough for wire drawing due to high lead content. The continued improvement of the anode material is critical to increase the life of the anode and the purity of the product, and to make electrowon copper suitable for most commercial uses.